The present invention is directed to pumping very viscous fluids which are difficult or impossible to pump with ordinary centrifugal pumps at efficiencies to be commercially practical. Such viscous fluids are non-Newtonian, in that, under laminar conditions, the viscosity is not a constant. The present invention will be described hereinafter in connection with one usage successfully pumped and that usage is digested sludge at about 14.3% by weight of organic solids; but, the invention is capable of use in pumping other viscous fluids, such as paints, plastics, foods such as tomato catsup, tomato paste, etc . . . Many of these fluids behave as a Bingham plastic or a borderline Bingham plastic in which the head requirements for elevation change and for velocity may be the same as for water, but the friction losses are substantially higher than for water. Unlike water flow which is usually turbulent when being pumped through pipelines, these viscous materials such as digested sludge of 10%, plus solids may experience laminar flow with friction losses many times greater than friction losses for water. Indeed, charts are available to determine a multiplication factor for digested sludge or untreated or primary and concentrated sludges where laminar sludge flow occurs and one wants to determine the friction losses for pumping sludge relative to friction losses for pumping water.
Sludge and other materials are sometime called "psuedoplastic" or borderline plastic materials, or, in some instances, Bingham plastic materials in which essentially no flow occurs unless the pressure is high enough to exceed a yield stress constant. In addition to the yield stress constant, another constant, which is the coefficient of rigidity, is also determined. With these constants determined, the pressure drop over a range of velocities may be found after calculating the Reynolds number and the Hedstrom number. A much higher velocity is needed for turbulent flow to occur for these viscous materials such as sludge; but, once turbulent flow is achieved, the pressure drop may be roughly thtt of water.
Sludge having a solids content of 10% or greater is a thixotropic material having a flow resistance depending on the length of time shearing and the intensity of shearing with the viscosity dropping at the time of shearing followed by a gradual recovery of viscosity when the shearing is stopped. Attempts have been made heretofore to assist centrifugal pumps in pumping sludge by agitating the sludge with motor driven impellers adjacent the pump inlet. While experimental pumping has been achieved of sludge of solids of 10% and greater using such auxiliary impellers with centrifugal pumps, the results have not been satisfactory to warrant commercial adoption thereof.
Sludge is accumulated in large quantities at waste water treatment plants and is often put into ponds or lagoons for long periods for storage and thickening prior to the sludge being transported to a disposal site or being utilized as a soil conditioner or fertilizer. The digested sludge increases in viscosity very quickly with being dehydrated. Sludge of about six percent solids by weight of organic material may be almost water-like in appearance and have a viscosity more akin to water in that it may be pumped successfully by commercial impeller driven pumps or other types of pumps. However, when the solids content is increased to ten percent, it is questionable as to whether or not impeller pumps can pump the sludge because of the viscosity of the material and its friction losses. When the solids content by weight is approximately ten percent, if there is any pumping at all using conventional pumps, it is accomplished only by the use of large quantities of power under very strenuous and demanding operating conditions. Little pumping is done of digested sludge having a ten percent solid, organic component. Digested sludge generally has large amounts of microorganisms which have digested the sludge and secreted material, many of which are like polysaccharides which are like a glue material which tends to stick and stick not only to pump parts but also to other organic organism materials.
Digested sludge of 10-20% solids is a generally thixotropic material and acts more like a semi-solid or a gelatin-like material than like water. Many of the microorganisms secrete carbon dioxide or other gases so there is a high concentration of dissolved gases in activated sludge. Further, the digestive bacteria in the sludge hydrate, i.e. swell with water and there is a considerable amount of bound water such that the sludge may be viscous although the density of the sludge fairly nearly approaches that of water rather than a higher density viscous material. Conventional impeller pumps quickly cavitate and fail to pump activated sludges having an organic solids content of eight percent or more organic. The cavitation is accompanied by the formation of gas or bubbles of gas which implode against the impeller causing the metal of the impeller to erode quickly while requiring higher amounts of power to drive the pump. Cavitation progresses until there is generally only bubbles and gases emanating from the pump. It appears that the viscous nature of tne digested sludge prevents the net positive suction head required to be met such that the pump is not receiving the sludge material required before it goes into cavitation. In some instances, a centrifugal pump when first lowered into a concentrated lagoon of sludge will pump sludge initially and then fail to pump and cavitate because the sludge will not flow into the pump inlet in sufficient quantities.
In other instances, vortex pumps have been used for pumping sludge. Such vortex pumps have a saucer shaped disk which have vanes on the inner curved surface of the disk. However, such vortex pumps are very inefficient and do not appear to pump sludge of ten percent or more solids.
Another problem with existing pump technology is that there may be a requirement for the use of water at the packing or bearings about the impeller shaft seal seal to flush the bearing and seals to prevent an ingress of sludge and inorganic material within the sludge from moving into the bearing and seal and destroying the packing and/or bearing. The use of water at such a seal is counter productive in that there may be as much as 15 to 20 gallons per minute used at the seal with the water leaking into the sludge, which is working in the opposite direction to all of the time and effort that has been expended to extract water from the sludge to make it a more solid for use as a fill. Within digester tanks, a vacuum or low pressure is created within the tank to draw off water vapor to concentrate the sludge. In some settling ponds, the sludge is graded by being pulled up a steep slope by a bucket or other mechanical device to allow water within the sludge to drain down the slope for collection and removal from the sludge. The water-like material withdrawn from the sludge is called "supernatant".
In large tanks of sludge, it is often the common practice where the solids content is very high, for example, 15% to 16% solids of organic material, to use power-operated buckets to scoop buckets of sludge for depositing into a truck for removal. At the 15% or 16% solids, the sludge appears to be solid whereas it is, in fact, a semi-solid or borderline plastic which has some attributes of liquid and some attributes of solids. When the organic solid materials reaches 20%, the dewatered activated sludge almost appears dry and will wobble and quake like a bowl of jelly. Even at this high percentage of solids, specific gravity of the sludge is not that much greater than water alone. Thus, sludge of a high solids content is a highly viscous and a very tacky adhesive-like material which has been found to be difficult to pump. There is a need to pump such sludge as well as other very viscous materials and also there is a need to improve existing pumping techniques for viscous materials.
Accordingly, a general object of the present invention is to provide a new and improved method and apparatus for pumping viscous materials.
Another object of the invention is to provide a new and improved method for pumping thixotropic materials.
A still further object of the invention is a new and improved method of and an apparatus for pumping sludge.
A more specific object of the invention is to provide a method and apparatus for pumping digested sludge having 10% solids or greater by weight of organic materials.
Other objects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings in which: